This disclosure generally relates to marking methods and systems. Embodiments are also related to developing tone reproduction curves that facilitate consistent and accurate printing from pitch to pitch on a photoreceptor and/or an intermediate transfer belt and/or other marking element.
This disclosure refers to marking as a process of producing a pattern, such as text and/or images, on a substrate, such as paper or transparent plastic. A marking engine performs the actual printing by depositing ink, toner, dye, or any other suitable patterning material on the substrate. For brevity, the word “ink” will be used to represent the full range of patterning materials, and is used interchangeably with the terms for other patterning materials.
A popular marking engine today is the xerographic marking engine used in many digital copiers and printers. In such a marking engine, a photoreceptor whose electrostatic charge varies in response to light is placed between an ink supply and the substrate. In xerographic systems, the ink is typically toner. A laser or bank of light emitting diodes is used to expose the photoreceptor to light to form an image of the pattern to be printed on the photoreceptor. In the simplest, monochromatic xerographic engines, toner is applied to the image to create a toner image on the photoreceptor, which toner image is then fused onto the substrate. In more complex systems, additional colors of toner are applied.
Color systems include Image On Image (IOI) systems and tandem systems. In an IOI system, such as that shown schematically in FIG. 1, the engine 10 includes plural primary color applying units 11 which deposit their inks on the photoreceptor 13, such as a belt, which includes multiple image forming areas that are hereafter called pitches 14. One of the pitches 14 of the photoreceptor 13 receives the first toner image in a first color, which remains on the photoreceptor 13 while a second toner image is created in a second color atop the first image in the same pitch 14, the first and second toner images remain on the photoreceptor while a third toner image is created in a third color atop the first and second images in the same pitch, and so forth. Once all of the toner images have been placed on the photoreceptor 13, they are transferred to the substrate, typically paper, and fused to the substrate. Furthermore, after the pitch 14 has passed one of the color applying units 11, the next pitch 14 comes into alignment with that unit 11, and the image forming process starts again in the next pitch 14.
In an embodiment of tandem system architecture, such as that shown in FIG. 2, the marking engine 20 includes multiple primary color applying units 21 which first deposit their inks on respective photoreceptors 22, typically drums, to form toner images, which are then deposited on the intermediate transfer belt (ITB) 23, which includes multiple pitches 24. Each toner image is transferred onto the ITB before the next toner image is formed. Like the IOI system, the toner images are fused once all for a given pitch have been deposited on the ITB.
In a variant of the tandem system shown in FIG. 2, each ink station can include an additional drum between the photoreceptor and the ITB. The additional drum accepts the toner image from the photoreceptor drum and deposits it on the ITB. The inclusion of the additional drum reduces the likelihood of toner of another color getting into a given ink source due to electrostatic interactions between the toner image on the ITB and the photoreceptor drum. Each of the printing architectures found in the marketplace has advantages, but all suffer from color reproduction problems.
It has been found that compensating for color variance throughout the color gamut of the color printer can be achieved by adjusting the ink mixture to produce gray level balance. This can be performed by printing one or more test patches based on particular requested gray levels, analyzing the output with a spectrophotometer, and generating a tone reproduction curve (TRC). The TRC is then used to alter the theoretical combination of ink to produce more accurate color with an actual combination.
When using cyan, magenta, yellow, and black inks to produce a process gray, TRCs can be used to more accurately produce a desired gray. If, for example, one desires a process gray of 128 cyan, 128 magenta, 128 yellow, and 0 black, but the marking engine used must employ 131 cyan, 127 magenta, and 130 yellow, and 0 black to achieve the desired result, TRCs can adjust the requested amounts so that the marking engine deposits 131 cyan, 127 magenta, 130 yellow, and 0 black, yielding the desired process gray. Preferably, a different TRC is used for each ink that a marking engine uses so that a CMYK marking engine will have four TRCs. TRCs can have different ranges of saturation values, such as 0 to 1, 0 to 100, or 0-255. Regardless of the input range and output range, all TRCs are used to adjust the amount of ink deposited by mapping an input value to an output value.